The present invention relates to an electron gun for the color picture tube comprising an electronic lens configuration capable of producing a high resolution and a superior convergence characteristic over the entire range of the phosphor screen of the color picture tube.
The resolution of the picture tube depends to a large measure on the diameter of the electron beam spot and the shape thereof. Specifically, a high resolution cannot be obtained unless the electron beam spot which is formed as a bright spot on the phosphor screen by the bombardment of an electron beam is small in diameter or near to the true circle.
In view of the fact that the electron beam track from the electron gun to the phosphor screen surface is lengthened with the increase in the deflection angle of the electron beam, however, if the optimum focus voltage is maintained to produce a truely circular electron beam spot small in diameter at the central part of the phosphor screen surface, an overfocused condition would result at the peripheral part of the phosphor screen, thereby making it impossible to produce a superior electron beam spot or a high resolution at the peripheral part.
In order to solve this problem, what is called the dynamic focus system has so far been employed in which the focus voltage is increased with the deflection angle of the electron beam to weaken the main lens electric field. This system, however, is not suitable for the driving of an in-line color picture tube as explained below.
Specifically, in an in-line color picture tube with three electron beam emitters arranged in alignment along the horizontal scanning direction, the horizontal-deflection magnetic field distribution and the vertical-deflection magnetic field distribution are distorted in pin-cushion and barrel forms respectively in order to produce a self-convergence effect, and therefore the electron beam that has passed this part is distorted into a non-circular form.
The phosphor screen, which is normally a laterally long rectangle with a long side in the direction of electron beam arrangement (horizontal direction), has an especially great distortion at the horizontal peripheral part.
FIG. 1 is a diagram for explaining the relationship between a four-pole lens magnetic field and electron beams. Numerals 1, 2 and 3 designate electron beams and numeral 4 a horizontal-deflection magnetic field.
A diagram for explaining the relationship between the horizontal-deflection magnetic field in the pin-cushion magnetic distribution and an electron beam is shown in FIG. 2, in which numeral 6 designates a two-pole magnetic field component, numeral 7 a four-pole magnetic field component, and numeral 9 an electron beam.
FIG. 3 shows a diagram for explaining the shape distortion of a beam spot, in which numeral 9H designates a high-brightness part (core) of the electron beam and numeral 9L a low-brightness part (haze part) thereof.
In FIG. 1, three electron beams 1, 2 and 3 advanced from the back of the page enter the horizontal-deflection magnetic field 4 in pin-cushion distribution and are deflected in the direction indicated by the arrow 5. Specifically, the horizontal-deflection magnetic field of pin-cushion distribution is considered to be comprised of the two-pole magnetic field component 6 as shown in FIG. 2(a) and the four-pole magnetic field component 7 as shown in FIG. 2(b). The two-pole magnetic field component 6 exerts the effect of deflection on the electron beam 9 in the direction shown by arrow 8.
The four-pole magnetic field component 7 exerts a self-convergence effect on three electron beams. As to a single electron beam 9, however, the horizontal scattering and the vertical convergence leads to a sectional form laterally long and flat.
The scattering effect works in such a direction as to cancel the overfocus of the electron beam spot as a result of a lengthened electron beam track with the increase in electron beam deflection angle, and therefore the optimum focus condition is maintained during the deflection period in the horizontal direction of the electron beam spot in an in-line color picture tube. In the vertical direction, however, the degree of overfocus is extremely increased by adding the above-mentioned convergence effect.
As a consequence, the electron beam spot formed at the cental part of the phosphor screen assumes a circular form as shown by "00" of FIG. 3, while the electron beam spot formed at the horizontal peripheral part is distorted into a non-circular form including a high-brightness core 9H and a low-brightness haze 9L. Especially, the great elongation along the vertical direction of the haze section 9L has an adverse effect on the focus characteristic.
In this case, if a conventional dynamic focus system is used, however, the function of the main lens is weakened uniformly either in horizontal or vertical direction, and therefore even if the haze section 9L is removed for the vertical direction, an underfocus condition is developed in the horizontal direction in spite an already-optimum focus condition, thereby increasing the horizontal diameter.
As a result, the electron beam spot is extremely lengthened horizontally for a reduced resolution in the horizontal direction.
A picture tube unit which has solved this problem and is capable of producing a high resolution over the entire range of the phosphor screen is disclosed in JP-A-62-58549.
FIG. 4 is a diagram for explaining the electron gun of a picture tube unit disclosed in the aforementioned patent publication, in which FIG. 4(a) is a general sectional view of the electron gun, FIG. 4(b) a front view of a first focusing electrode, and FIG. 4(c) a front view of a second focusing electrode. Numerals 10a, 10b and 10c designate cathodes, numeral 110 a control electrode, numeral 120 an accelerating electrode, numeral 130 a first focusing electrode, numeral 140 a second focusing electrode, numeral 150 an anode, and small alphabetical characters affixed to the numerals 110 to 150 respective electron beam passage apertures. Character C designates an electron gun axis (coinciding with the tube axis), character L.sub.M a main lens, and characters S.sub.1 to S.sub.4 distances of the side electron beam passage apertures of each electrode from the electron gun axis C (coinciding with the center electron beam).
In FIG. 4, at least the accelerating electrode 120, the first focusing electrode 130 and the second focusing electrode 140 are arranged sequentially along the tube axis between the control electrode 110 and the anode 150. Vertical electron beam passage apertures 130d, 130e and 130f are arranged at the end of the first focusing electrode 130 on the side of the second focusing electrode 130, and the lateral electron beam passage apertures 140a, 140b and 140c at the end of the second focusing electrode 140 near to the first focusing electrode 130.
The unit has voltage application means for applying a first predetermined focus voltage to the first focusing electrode 130, a predetermined high voltage to the anode 150, and a dynamic voltage changing to a higher value than the first focus voltage with the increase in the deflection angle of the electron beam to the second focusing electrode 140.
In this configuration, at the time when the horizontal deflection is zero, that is, when the first focusing electrode 130 and the second focusing electrode 140 are both at the same potential, the electron beams are not substantially affected regardless of whether the electron beam passage apertures of the electrodes are longitudinal (long in the vertical direction, that is, in the direction perpendicular to the horizontal direction) or in the lateral direction (long in the horizontal direction).
A potential difference is thus caused between the second focusing electrode 140 and the another 150, and the resulting three main lenses L.sub.M formed at this point cause three electron beams to be focused at optimum convergence at the central part of the phosphor screen.
With the increase in the horizontal deflection angle, the potential of the second focusing electrode 140 becomes higher than that of the first focusing electrode 130, thereby generating a four-pole lens electric field between the two electrodes by the longitudinal electron beam passage apertures 130d, 130e and 130f and lateral electron beam passage apertures 140a, 140b and 140c.
Also, the reduction in the potential difference between the second focusing electrode 140 and the anode 150 weakens the function of the main lenses.
FIGS. 5 and 6 are diagrams for explaining the effect that the four-pole lens electric field has on the electron beam. In these diagrams, for facilitating understanding, a flat electrode 213 having a single longitudinal electron beam passage aperture 212 is arranged in opposed relationship with a flat electrode 215 having a single lateral electron beam passage aperture 14, and potentials V.sub.1 and V.sub.2 are applied to them.
In FIG. 5, the four-pole lens electric field formed between the two electrodes under the voltage condition satisfying V.sub.1 &lt;V.sub.2, as shown in FIG. 6, is positive in potential at upper and lower parts and negative at right and left parts with respect to the central part. As a result, electric lines of force are generated in the direction indicated by arrow 216, so that the electron beam 217 assumes a longitudinal section under attractive and repulsive forces in the direction indicated by arrows 216.
This is in contrast with the case in which the electron the beam that has passed the deflection magnetic field assumes a lateral section due to the four-pole magnetic field components shown in FIG. 2(b). It is thus possible to prevent the electron beam from flattening laterally by the two fields offsetting with each other.
Further, with the increase in deflection angle, the focusing function of the main lens is reduced as mentioned above, and therefore overfocusing due to the deflection of an electron beam spot is prevented at the same time. An electron beam spot small in diameter and almost true in roundness is generated even along the peripheral parts of the phosphor screen.
Also, in FIG. 4, the application of a dynamic focus voltage to the second focusing electrode 140 is liable to cause a displacement of convergence between three electron beams. As a measure against this, the relationship S.sub.4 &lt;S.sub.3 &lt;S.sub.1 &lt;S.sub.2 is maintained, where S.sub.1 is the distance of the side electron beam passage apertures 110b, 110c, 120b and 120c of the control electrode 110 and the accelerating electrode 120 from the electron gun axis C (coinciding with the electron beam and tube axis), S.sub.2 is the distance of the side electron beam passage apertures 130b and 130c at the end of the first focus electrode 130 near to the accelerating electrode 120 from the electron gun axis C, S.sub.3 is the distance of the side electron beam passage apertures 130e, 130f, 140b and 140c at the opposed ends of the first focusing electrode 130 and the second focusing electrode 140 from the electron gun axis C, and S.sub.4 is the distance of the side electron beam passage apertures 140e, 140f, 150b and 150c at the opposed ends of the second focusing electrode 140 and the anode 150 from the electron gun axis C.
As a result, the orbital axis of the side electron beams is maintained constant with respect to the change in dynamic voltage, thereby minimizing the misconvergence of the side electron beam and the electron beam spot distortion caused by the distortion of the deflection magnetic field.
In the aforementioned prior art, in changing the dynamic voltage of the second focusing electrode, the distance of three electron beam passage apertures between the control electrode and the first focusing electrode, between the first focusing electrode and the second focusing electrode, and between the second focusing electrode and the anode is changed in order to concentrate the three electron beams emitted in lateral alighment from the cathode on the screen surface.
This makes it necessary to use a special electron gun assembly jig for matching the electron beam passage aperture distances S1, S2, S3 and S4 with each other and the longitudinal electron beam passage aperture of the first focusing electrode with the lateral electron beam passage aperture of the second focusing electrode in assemblying the respective electrodes, thus making the assembly work extremely difficult and unsuitable for mass production.